ChemInform Abstract: Tailoring Nanostructures in Micrometer Size Germanium Particles to Improve Their Performance as an Anode for Lithium Ion Batteries.

Abstract

A facile and scalable single-step approach is employed to synthesize a bulk germanium electrode, which consists of nanoscale Ge-grains in ∼5 μm porous powders. This three-dimensional Ge electrode exhibits superior specific capacity (∼1500 mA h g(-1)) and cyclic performance, attributed to its unique lithiation/delithiation processes.

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This journal is ©The Royal Society of Chemistry 201 4 Chem. Commun., 2014, 50, 3713--3715 | 3713
Cite this: Chem. Commun., 2014,
50,3713
Tailoring nanostructures in micrometer
size germanium particles to improve their
performance as an anode for lithium ion
batteries†
Fu-Sheng Ke,*
ab
Kuber Mishra,
a
Lauryn Jamison,
a
Xin-Xing Peng,
c
Shu-Guo Ma,
a
Ling Huang,
c
Shi-Gang Sun*
c
and Xiao-Dong Zhou*
a
A facile and scalable single-step approach is employed to synthe-
size a bulk germanium electrode, which consists of nanoscale
Ge-grains in B5lm porous powders. This three-dimensional Ge
electrode exhibits superior specific capacity (B1500 mA h g
1
) and
cyclic performance, attributed to its unique lithiation/delithiation
processes.
The next-generation electric vehicles necessitate the develop-
ment of lithium batteries with high-energy and high-power
density. Generally, the power density of a lithium ion battery
is limited by its negative electrode. The current primary choice
of the anode material is graphite due to its cost effectiveness,
but graphite exhibits a low energy density (372 mA h g
1
) and
poor rate-performance for use in future electric vehicles. As a
consequence, high performance negative electrode materials,
in particular Si (4200 mA h g
1
)
1–4
and Sn (994 mA h g
1
),
5–8
have been extensively explored because of their superior elec-
trochemical properties, such as high capacity, moderate
potential vs. Li/Li
+
, low cost, and environmental benignity.
The primary challenge to achieving stable performance of
Si- or Sn-based anodes originates from the large volume change
(260% for Sn and 320% for Si) occurring during insertion/
extraction processes of lithium ions. The repeated expansion/
contraction results in the pulverization of anode particles,
leading to the loss of electric contact between anode particles
and the current collector.
To mitigate the detrimental effects of volume changes,
several approaches have been reported, including the utiliza-
tion of a matrix (like carbon or graphene) into the active anode
materials to form Sn composites, the synthesis of intermetallic
compounds that can buffer volume fluctuation, and the design
of unique nanoarchitecturing morphology of the Si- or
Sn-based electrodes. Germanium (1620 mA h g
1
) has gained
attention
9–16
during the past few years for its nearly isotropic
lithiation,
14
which results in weak anisotropy of the lithiation
strain as shown from a recent in situ TEM study.
12
In addition,
Ge exhibits higher electronic conductivity and Li diffusivity
than Si, which potentially enables a high-rate performance of
the Ge-based anode. Although GeO
2
/Ge/C has shown interest-
ing reversible performance,
17
the question still remains as to
whether or not a bulk Ge electrode can be prepared without
arduous effort to build complicated electrode structure. In this
communication, we report our recent research on synthesis of
nano/microstructure–electrochemical property relationships in
a Ge electrode fabricated using a facile and scalable approach.
The initial research focused on the morphological evolution
of GeO
2
particles as a function of reduction temperature.
Detailed experimental procedures can be found in the ESI.†
The morphology of reduced Ge particles was analysed by using
scanning electron microscopy (SEM). Shown in Fig. 1 are SEM
images of the specimens reduced at 450 (Fig. 1a and 1b) and
600 1C (Fig. 1c and 1d) for 10 h. The SEM image of initial GeO
2
is shown in Fig. S1 (ESI†) for comparison, which consists
of a dense agglomeration of GeO
2
grains in the range of
100–200 nm. Upon reduction, it is found that hexagonal GeO
2
(24.75 cm
3
mol
1
) undergoes volume shrinkage to form cubic
Ge (13.63 cm
3
mol
1
). Indeed, porous Ge particles (Fig. 1a) were
obtained at 450 1C, comprised of B100 nm Ge grains and a
large number of nanosized pores (Fig. 1b). These pores can be
filled with the liquid electrolyte to facilitate diffusion of lithium
ions, while the size of B100 nm was reported to be capable of
tolerating lithiation/delithiation stress.
12
The as-synthesized
nanograins and nanopores are ‘‘self’’-assembled to form micro-
metre sized Ge particles, which are ideal for the fabrication of a
bulk electrode for lithium ion batteries. In contrast, reduction
at T= 600 1C results in not only the elimination of nanopores
(Fig. 1c), but also the growth of Ge grains (Fig. 1d), indicating
a
Department of Chemical Engineering, University of South Carolina, Columbia,
SC 29208, USA. E-mail: zhox@cec.sc.edu; Tel: +1-803-777-7540
b
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072,
PR China. E-mail: kefs@whu.edu.cn
c
State Key Laboratory of Physical Chemistry of Solid Surfaces,
Department of Chemistry, Xiamen University, Xiamen 361005, PR China.
E-mail: sgsun@xmu.edu.cn
†Electronic supplementary information (ESI) available: Experimental details,
XRD results, and SEM images. See DOI: 10.1039/c4cc00051j
Received 3rd January 2014,
Accepted 11th February 2014
DOI: 10.1039/c4cc00051j
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the onset of the sintering process at 600 1C for Ge (melting
point: 938 1C) that involves densification and grain growth. The
structure shown in Fig. 1c is not expected to be favourable for
battery electrodes. X-ray diffraction measurements (Fig. S2,
ESI†) show that GeO
2
can be reduced to Ge TZ450 1C in the
presence of hydrogen.
Electrochemical measurements were carried out to validate our
hypothesis on the aforementioned structure–electrochemical
property relationships by employing Ge powders reduced at 450
and 600 1C as the anode material. Fig. 2a shows the alloying/
dealloying (lithiation/delithiation) voltage profiles for the 1st,
2nd, 10th, 20th and 40th cycles at a constant current density of
50 mA g
1
. There exist multiple alloying plateaus while a single
delithiation plateau at B0.48 V vs. Li/Li
+
overlapping with each other
for all cycles (all potentials mentioned hereafter are referred to Li/Li
+
).
The initial alloying and dealloying capacities are B1920 mA h g
1
and
1450 mA h g
1
(Fig. 2b) in Ge electrodes consisting of nanopores
(reduction at 450 1C) respectively, yielding an initial Coulombic
efficiency of 76%, which is similar to the initial performance of the
Ge electrode synthesized at 600 1C(Fig.2candd).
The reason why the first alloying capacity is larger than the
theoretical capacity of Ge (ca. 1600 mA h g
1
)isbecauseofthe
decomposition of the electrolyte and the formation of a solid
electrolyte interphase layer, both of which can cause the irreversible
capacity visible in the first cycle. Comparing Fig. 2b and d, the
electrode consisting of nanopores and fine Ge grains exhibits
excellent cycleability. Fig. 2b illustrates that this type of Ge electrode
can deliver a charge capacity of B1500 mA h g
1
up to 40 cycles
with a capacity retention of 99%. This performance is much better
than that of the Ge electrode comprised of dense particles, which
retainsonly42%ofitschargeover40cycles(Fig.2d).
Fig. 3 shows specific capacity vs. cycle number for the porous
Ge electrode at rates of 100 and 800 mA g
1
, which exhibits an
alloying capacity of 1300 and 1100 mA h g
1
, respectively, and
the capacity is stable for up to 40 cycles. Further research is
being carried out to coat a thin layer carbon on Ge grains to
improve capacity and capacity retention at substantially high
rates (e.g. 410 C).
The differential capacity analysis has long been known for
being capable of assessing the mechanisms of the changes in
the voltage profile.
18,19
In differential capacity curves, each
peak generally represents a reaction or phase transition in
the active material. Fig. 4a shows the differential capacity plots
of the 1st, 2nd, 20th and 40th cycles for the electrode. Peaks
shown in Fig. 4a are corresponding to the alloying reactions
Ge + xLi -GeLi
x
, which occur at 0.20, 0.33 and 0.49 V, and
dealloying reactions initially at 0.45 and 0.62 V during the initial
Fig. 1 SEM images of Ge particles reduced at 450 1C (a and b) and 600 1C
(c and d) with different magnifications.
Fig. 2 Electrochemical properties bulk Ge electrodes prepared from the
thermal reduction of GeO
2
at 450 1C (a and b) and 600 1C (c and d). (a)
Voltage profile of the electrode reduced at 450 1C, (b) specific capacity vs.
cycle number of the electrode reduced at 450 1C, (c) voltage profile of
the electrode reduced at 600 1C, and (d) specific capacity vs. cycle number
of the electrode reduced at 600 1C. The charge–discharge rate was
50 mA g
1
.
Fig. 3 Specific capacity versus cycle number for nano/microstructure Ge
electrodes under 100 mA g
1
and 800 mA g
1
.
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process (GeLi
x
-Ge + xLi). After several cycles, alloying peaks are
centered at 0.20 and 0.40 V, suggesting that the porous Ge electrode
undergoes a two-step alloying reaction, likely forming Li
15
Ge
4
(1384 mA h g
1
)andLi
22
Ge
5
(1620 mA h g
1
).
12,16
Unlike Si-
20
and Sn-based anodes,
21
differential capacity plots for Ge
electrodes exhibit only one peak during delithiation, implying
this dealloying process, Li
x
Ge -Ge + xLi is one step, thus in
favour of reversibility.
Recent in situ TEM studies on the reversible expansion and
contraction of the Ge electrode revealed that Ge nanoparticles
(B160 nm) were able to sustain large volume changes during
cycling. These nanoparticles expanded and shrank instantly in a
uniform manner, which was attributed to the isotropic nature of
lithiation.
12
In our research, the nanopores, therefore, are pivotal in
accommodating isotropic volume expansion during the lithiation
process. The underlying significance of the presence of nanopores
is also shown in Fig. 4b, in which the intensities of the peaks
decrease during the cycling process. In the cycling process, a
decrease in peak height suggests that the number of available sites
to host lithium at these voltages decreases. The heights of the peaks
shown in Fig. 4a remain nearly constant, indicating that the
number of available sites for lithium occupancy is nearly constant.
Regarding lithiation composition, although Liang et al. reported
the formation of Li
15
Ge
4
(1384 mA h g
1
) in their studies, we find
that the specific capacity (B1500 mA h g
1
)andtheexistenceof
two peaks (0.2 and 0.4 V) during lithiation provide evidence for the
presence of Li
22
Ge
5
(1620 mA h g
1
)inourresearch.Thestable
performance shown in Fig. 2b suggests that Li
22
Ge
5
undergoes
uniform expansion and contraction during cycling.
In summary, a facile and scalable thermal reduction
approach was used to synthesize bulk germanium electrodes,
comprised of nanosized pores and Ge-grains in B5mmporous
powders. These nanopores and Ge grains assemble to form an
ideal electrode structure for lithium ion batteries, which yield
high capacity (93% of the theoretical value), high capacity
retention (99%) during cycling, and high-rate performance.
The presence of a large number of nanopores is vital to achieve
a high capacity and capacity retention. The superior electroche-
mical properties of porous bulk Ge electrodes in our research
demonstrate that this type of structure is promising in the
development of anode materials for future electric vehicles.
This work was partially supported by the start-up fund from
the University of South Carolina and the Solid State and
Materials Chemistry Program of the Division of Materials
Research at the National Science Foundation under Grant
Number DMR-1006113. XXP was supported by NFFTBS (No.
J1210014) as a visiting student at the USC. FSK acknowledges
the New Faculty Startup Fund of Wuhan University.
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consisting of nanopores and (b) Ge electrode comprised of dense grains.
Plots in a wider potential window can be found in Fig. S3 (ESI†).
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